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<br />The chance of embankment overflow is smaller .for the higher embankments. <br />Therefore, the cost to repair damages should decrease (fig. 6) with <br />increasing bridge lengths and increasing embankment elevations. <br /> <br />Data Analyses <br /> <br />Magnitude and Frequency of Floods <br /> <br />Flood frequency is computed using the best available procedure <br />using national, regional, or state reports. At this time, an extensive <br />set of reports has been written by the U.S. Geological Survey covering <br />flood-frequency analysis in each of the various states. These include <br />both large and small watersheds. The United States Water Resources <br />Council (1977) has published a procedure for developing flood frequency <br />at gaged watersheds and is currently working on recommendations for <br />ungaged sites. Various state agencies have developed methods for use <br />within the state. The sources of this information are well known to <br />hydrologists and hydraulic engineers within the state and is generally <br />available from the U.S. Geological Survey's district office. <br /> <br />Once a flood-frequency relation has been computed, a series of <br />floods can be selected for use in the risk analysis (column 1 of table 1). <br />In the risk analysis, a numerical integration is performed involving <br />floods of various frequencies. The more floods that are used the more <br />accurate the integration. However, the number of computations increase. <br />The floods are selected from the flood-frequency relation for various <br />exceedance prObabilities. <br /> <br />1. The first flood is one at which, if exceeded, some type of <br />damage will occur such as backwater, traffic delay, or erosion damage. <br />It will probably be at least the bankfull stage. <br /> <br />2. The base flood, the I-percent-chance flood, is used because the <br />entire stream crossing--the bridge, embankment and roadway--is required <br />to pass the I-percent-chance flood. <br /> <br />3. Select at least two intermediate floods between the first flood <br />and the I-percent-chance flood. <br /> <br />4. Select several large floods which would be expected to cause <br />damage in high risk areas that include critical facilities such as <br />hospitals, rest homes, and so forth. Such floods could cause major <br />traffic delays and incur large costs in repairing the highway crossing. <br />The 0.2- and 0.5-percent floods are usually such floods. <br /> <br />Hydrographs <br /> <br />In addition to the selection of a sequence of peak discharges, a <br />flood hydrograph with that peak is needed to compute the duration of <br />flow over the road. In some instances the local scour potential is <br />increased for the larger hydrographs, that is, the longer the water is <br />up the more extensive and severe the local scour might be. <br /> <br />24 <br />